Fig. 6. (A ) Respiratory variations of the plethysmographic waveform variation (PWV) plotted against arterial systolic pressure variations (SPV). (B ) Respiratory variations in the pulse oximetry photoplethysmographic waveform amplitude (dPOP) plotted against the arterial pulse pressure variation (PPV) in a scatterplot. The circles represent the values of the two variables in all patients at the same step of blood volume reduction and are presented with the same color. Each color of the regression lines represents the same step of blood withdrawal. Two values from the PPV and dPOP plot were removed for lack of space. Correlations within both comparisons improved with increased stages of blood withdrawal. CBV = circulating blood volume.